This invention relates to a method for producing a carbonaceous powder which may be used as dielectric fine particles to be dispersed in an electrorheological fluid. The fluid having the carbonaceous powder of the present invention dispersed therein may exhibit excellent electrorheological properties.
An electrorheological fluid is a fluid which may under go a rapid, reversible change in its apparent viscosity through application of an electric field to exhibit a so called Winslow effect. An electrorheological fluid is generally produced by dispersing dielectric particles in an electric insulating oily medium. Such a characteristic of the electrorheological fluid has been known for many years, and the electrorheological fluid has now become a promising material for many applications including clutches, valves, vibration dampers, and the like.
An electrorheological fluid is generally required to exhibit a large change in its viscosity upon application of a weak electric field and minimize the electric current in the fluid for realizing a higher energetic efficiency. It is also required that the electrorheological fluid may retain its colloidal state without precipitation of the solid particles in the oily medium. It is further required that the properties of the fluid may not degrade (e.g. not lower viscosity change and not raise current value) after prolonged use, and that the properties may hardly be varied by the change of temperature in use. Quick response to an electric field applied are also required.
Conventional electrorheological fluids include dispersions of high dielectric liquid such as water and alcohols, absorbent solid particles such as silica gel, starch, and cellulose in an oily medium having a superior electric insulation such as transformer oil, spindle oil, and chlorinated paraffin. Such fluids are disclosed in U.S. Pat. Nos. 2,886,151 and 3,047,507, and Japanese Patent Application Kokai Nos. 53(1978)-17585, 53(1978)-93186, 61(1986)-44998, 61(1986)-259752, 62(1987)-95397, 1(1989)-207396 and the like. There have also been disclosed dispersions wherein the solid particles are coated with a polymer to overcome various disadvantages caused by the use of water-impregnated solid particles including degradation of properties after prolonged use and poor performance at a high temperature. Such fluids are described in Japanese Patent Application Kokai Nos. 47(1972)-17674 and 63(1988)-97694.
Such conventional electrorheological fluids, however, failed to exhibit performances sufficient for practical use.
In view of such a situation, the inventors of the present invention have proposed a carbonaceous powder which may be used as dielectric particles to be dispersed in a medium exhibit required electrorheological performances. See Japanese Patent Application Kokai No. 3(1991) -279206. This carbonaceous powder has been subjected to a carbonization to a controlled degree, and does not require impregnation with a highly dielectric liquid. More illustratively, the inventors of the present invention have disclosed that the carbonaceous powder which may be used as dielectric powder to be dispersed in an electrorheological fluid may preferably have the following properties:
C/H value which is the ratio in number of carbon atoms to hydrogen atoms in the elemental analysis in the range of from 1.70 to 3.50, and preferably from 2.20 to 3.00;
weight loss determined by Thermal Gravimetric Analysis (hereinafter referred to weight loss value by TGA) at a temperature range of from 400.degree. to 600.degree. C. in nitrogen atmosphere of from 0.5 to 13.0% by weight; and
maximum particle size of 50 .mu.m and mean particle size in the range of from 0.5 to 40 .mu.m as determined by Coulter Counter having an aperture tube of 50 .mu.m.
In the above-mentioned Japanese Patent Application Kokai No. 3(1991) -279206, the inventors of the present invention disclosed that the carbonaceous powder having such properties may be produced by a method comprising the steps of
subjecting a starting organic material such as coal, coal tar, coal tar pitch, liquefied coal, coke, petroleum, petroleum tar, petroleum pitch, or a resin to a heat treatment at a maximum temperature in the range of from 300.degree. to 800.degree. C., in an autoclave, a kiln, or an electric furnace to reform the material and control its C/H value as well as its content of the components which are volatilizable at a temperature in the range of from 400.degree. to 600.degree. C. PA0 subjecting a starting material comprising an organic compound selected from the group consisting of coal, coal tar, coal tar pitch, liquefied coal, coke, petroleum, petroleum tar, petroleum pitch, and resins to a heat treatment at a maximum temperature of 300.degree. to 800.degree. C. to produce a carbonaceous material; PA0 pulverizing and classifying the material to produce carbonaceous particles having a mean particle size of 0.5 to 40 .mu.m and a maximum particle size of up to 50 .mu.m; and PA0 subjecting the particles to an additional treatment at an elevated temperature and/or a reduced pressure, said additional treatment being carried out at a temperature lower than said maximum temperature of the heat treatment (the heat treatment before the pulverization to produce carbonaceous particles having a mean particle size of 0.5 to 40 .mu.m and a maximum particle size of up to 50 .mu.m). PA0 subjecting a starting material comprising an organic compound selected from the group consisting of coal, coal tar, coal tar pitch, liquefied coal, coke, petroleum, petroleum tar, petroleum pitch, and resins to a heat treatment at a maximum temperature of 300.degree. to 800.degree. C. to produce carbonaceous material; PA0 pulverizing and classifying the material to produce carbonaceous particles having a mean particle size of 0.5 to 40 .mu.m and a maximum particle size of up to 50 .mu.m; PA0 subjecting the particles to an additional treatment at an elevated temperature and/or a reduced pressure, said additional treatment being carried out at a temperature lower than said maximum temperature of the heat treatment (the heat treatment before the pulverization to produce carbonaceous particles having a mean particle size of 0.5 to 40 .mu.m and a maximum particle size of up to 50 .mu.m); and PA0 subjecting the particles to a further pulverization treatment.
The time required and the temperature in the heat treatment are property selected according to the starting material. When a coal tar pitch is used as a starting material, the heat treatment is processed at a temperature of 400.degree. to 600.degree. C., and at least for five hours. In case of a phenol resin as a starting material used, the heat treatment is processed at the temperature of 500.degree. to 600.degree. C., and at least for 3 hours.
Optionally pulverizing the resulting carbonaceous material with such means as ball mill or jet mill and classifying the material by such means as sifting and air classification.
However, it has been found that the fluid prepared by dispersing such a carbonaceous powder to a certain concentration may frequently exhibit electrorheologically unstable performance, for example, fluctuation in viscosity and electric current upon application of a relatively high voltage for a period as short as approximately ten minutes. Such a lack in stability upon application of a relatively high voltage is quite detrimental for practical use, and therefore, stabilization of the electrorheological properties upon application of a relatively high voltage was critical for practical use of the fluid.
Accordingly, an object of the present invention is to further improve the above-mentioned carbonaceous powder which may be used as dielectric particles for an electrorheological fluid proposed by the present inventors, and provide a method for producing a carbonaceous powder which may be dispersed in a medium to constitute an electrorheological fluid which does not exhibit fluctuation in viscosity or electric current upon application of a high voltage.
Up to now, mechanisms for the development of electrorheological effects are not yet fully revealed. It is, however, believed that, upon application of an electric field to the electrorheological fluid, the particles dispersed therein undergo polarization, and the polarized particles are attracted to each other by electrostatic attraction to result in an increased apparent viscosity of the fluid. Therefore, performance of an electrorheological fluid comprising a dispersion of dielectric particles in an electrically insulating oily medium would definitely be influenced by nature of the particle surface.
From such a point of view, we estimated that the above-described instability in viscosity and electric current of the fluid having the above-described carbonaceous powder dispersed therein is caused by the phenomenon as described below.
In the production of the carbonaceous powder, the organic material is subjected to a heat treatment in order to control its C/H value as well as its weight loss value by TGA to the above mentioned ranges. However, volatile components having low boiling points are not fully expelled from the carbonaceous material in this heat treatment, and when the carbonaceous material is subsequently pulverized, the parts of the material containing the residual volatile components having low boiling points, which were in the interior of the carbonaceous material, were exposed as exterior surface of the resulting particles. Consequently, the thus produced carbonaceous particles had particle surfaces wherein the part containing the residual volatile components having low boiling points are inconsistently distributed. Such an inconsistency of the particle surface resulted in poor electrorheological stability of the fluid in which the particles are dispersed to show the fluctuated viscosity and the electric current.
On the basis of such an estimation, the inventors of the present invention carried out a further investigation and found that the above-described instability of the electrorheological properties may be improved by subjecting the carbonaceous particles to an additional treatment wherein the content of the volatile components having low boiling points in the particle surface is controlled, and that such an additional treatment should be carried out after the pulverization of the carbonaceous material to the final particle size at which the carbonaceous powder is used, namely, dispersed in the electric insulating oil. More illustratively, after the heat treatment of the organic material carried out to control the C/H value and the weight loss value by TGA and the subsequent pulverization and classification, the carbonaceous powder is subjected to an additional treatment at an elevated temperature and/or a reduced pressure carried out at a temperature lower than the maximum temperature of the previous heat treatment (the heat treatment is processed before the pulverization to produce carbonaceous particles having a mean particle size of 0.5 to 40 .mu.m and a maximum particle size of up to 50 .mu.m) to thereby efficiently reduce or remove the low boiling components in the particle surface. The volatile components of the surface have been sufficiently removed. The fluid having dispersed therein the resulting carbonaceous particles having sufficiently consistent, homogeneous surface was found to exhibit quite stable electrorheological properties upon application of a relatively strong electric field. The change of viscosity through application of an electric field is increased without increasing the current. It was also found that the weight loss value by TGA of the thus obtained carbonaceous powder at a temperature range of from a room temperature to 200.degree. C. can be used as a value representing the content of such volatile components having low boiling points. It is to be noted that the additional treatment is to be carried out after the pulverization and the classification of the carbonaceous particles to the final particle size at which the carbonaceous particles are used as the dielectric particles dispersed in the electrorheological fluid, namely, a maximum particle size of up to 50 .mu.m and a mean particle diameter of 0.5 to 40 .mu.m. No improvement in stability would be achieved when such an additional treatment is carried out before the pulverization and the classification of the carbonaceous particles to the final particle size.
However, the inventors of the present invention found out another unexpected problem to be obviated. The fluid having dispersed therein the carbonaceous particles which had been subjected to such an additional treatment at an elevated temperature exhibited an increased apparent viscosity with no application of the electric field (hereinafter referred to as initial viscosity). Such an increase in the initial viscosity of the fluid is quite unfavorable for practical use of the electrorheological fluid.
Initial viscosity of an electrorheological fluid is a quite important property. If the carbonaceous powder exhibits a low initial viscosity upon dispersion in the medium, it would be possible to increase the quantity of the carbonaceous powder dispersed in the fluid to a level that would not exceed the predetermined initial viscosity. An increased quantity of the carbonaceous powder dispersed in the fluid can realize a larger change in electrorheological properties of the fluid upon application of an electric field. Therefore, a carbonaceous powder to be dispersed in an electrorheological fluid as dielectric particles is required to show a low initial viscosity upon dispersion in a fluid.
Increase in the initial viscosity of the electrorheological fluid may occur when the above-mentioned additional treatment is carried out at an excessively high temperature or temperature elevation rate. Such an increase in the initial viscosity is believed to have been caused by the following phenomena.
If the particle surface is exposed to an atmosphere at a temperature higher than the melting point of the components constituting the particle surface in the additional treatment at an elevated temperature and/or reduced pressure, the particle surface would melt and the particles would become fused to each other. Such a fusion of the particles would result in an increased particle size to accelerate precipitation of the particles in the dispersion. Such a fusion of the particles would also produce particles of complicated configuration, for example, in dumbbell shape leading to an increased initial viscosity.
The inventors of the present invention have found that the problem caused by the fusion of the particles may be obviated by subjecting the carbonaceous powder to a still further treatment wherein the carbonaceous powder is pulverized so that the particles which became fused in the additional treatment would be separated, and the carbonaceous powder will restore the particle size before the additional treatment. The initial viscosity will then be decreased to its original level before the additional treatment, and the problem of the increased initial viscosity is thereby obviated.
Therefore, it is the object of the present invention to further improve an excellent carbonaceous powder which may be used as dielectric particles for an electrorheological fluid which were previously proposed by the present inventors, and provide a method for producing a carbonaceous powder which can limit a fluctuation in viscosity (the amount of a fluctuation in viscosity measured for ten minutes and represented by (maximum value-minimum value)/average value.times.100%) and a fluctuation in current value (the amount of a fluctuation in current value measured for ten minutes and represented by (maximum value-minimum value)/average value.times.100%) to less than 6% upon application of a relatively strong electric field of 3 kV/mm to the fluid and which does not increase the initial viscosity of the carbonaceous powder.